Electrolytic cell with reference electrode

ABSTRACT

A reference electrode device is provided for a high temperature electrolytic cell used to electrolytically recover uranium from spent reactor fuel dissolved in an anode pool, the device having a glass tube to enclose the electrode and electrolyte and serve as a conductive membrane with the cell electrolyte, and an outer metal tube about the glass tube to serve as a shield and basket for any glass sections broken by handling of the tube to prevent their contact with the anode pool, the metal tube having perforations to provide access between the bulk of the cell electrolyte and glass membrane.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy andThe University of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates to reference electrodes for use in hightemperature environments which may involve high thermal shock conditionsand more particularly to reference electrodes with protective structuresover the electrode membranes. Specifically, the invention relates toreference electrodes useful in mixed halide salts as high temperatureelectrolytes in electrolytic cells for the electrorefining of spentreactor fuels.

In the electrorefining of two or more metals involving the selectivedeposition of at least one of the metals, a reference electrode isuseful as a standard potential in the study of the process and tocontrol the cathode potential without dependence on changing anodepotentials. Control of the cathode potential is important in determiningwhich metal of the metal mixture is being deposited on the cathode at aparticular stage of the process. One new reactor system underdevelopment and evaluation by the U.S. Department of Energy involves theuse of a metal fuel of U-Pu-Zr. Processing of spent fuel is carried outelectrolytically during which U and Pu may be separately deposited onthe cathode. The cell electrolyte is a mixed halide salt and the celloperating temperature is in the order of about 500° C. The anode is apool of liquid cadmium with the spent fuel below the electrolyte in thecell. The cell is operated in an inert atmosphere of argon with lessthan 2 ppm of water vapor and less than 2 ppm of oxygen. An earlierversion of the cell is disclosed in U.S. Pat. No. 4,596,647 which ishereby incorporated herein by reference.

The reference electrode previously utilized and tested in theexperimental electrolytic cell has been an alumina tube with a smallhole drilled in the bottom for ionic access between the cell electrolyteand the electrolyte of the reference electrode. The reference electrodewas based on a Ag/AgCl electrode with added electrolyte usually havingthe same composition as the nonreactive components of the main saltelectrolyte of the electrolytic cell. The silver electrode was a wirewith a lower end formed into a small coil or helix and was primarilyretained in the tube except for an exposed upper end section forelectrical connection. In general, this reference electrode had a largedrift rate and required frequent regeneration by an anodizing step.These problems appeared to be due to a diffusive transport of silverchloride through the small opening (although packed with yttria fiber)at the lower end of the reference electrode.

Accordingly, one object of the invention is an electrolytic cell with areference electrode operable for an extendable period of time attemperatures in the order of 500° C. A second object of the invention isan electrolytic cell with a reference electrode operable in the cellcontaining a molten chloride salt electrolyte and a liquid metal anodepool containing uranium, plutonium, zirconium, sodium and other metals.Another object of the invention is a reference electrode with a glasselectrode which is usable in a high thermal shock environment. These andother objects will be readily apparent from the following description.

SUMMARY OF THE INVENTION

Briefly, the invention is directed to a reference electrode device formounting in a housing of an electrolytic cell and extending into anelectrolyte layer in the interior of the cell, the device comprising anelongated glass tube extending from the housing into the interior of thecell and having a lower closed-ended section with ionic conductivity forexposure to the cell electrolyte, a metal electrode supported in theglass tube and in separated juxtaposition with the closed end, anelectrolyte in the lower section in contact with the electrode, and anelectrical insulator separating the electrode and glass tube. Thereference electrode device of the invention further includes anelongated metal tube extending from the housing over the glass tube as aprotective shield or cover and basket with the metal tube having aperforated side wall in contact with the cell electrolyte and means areprovided for supporting the glass tube within the metal tube, and forsupporting the metal tube in the cell housing. The one-piece glass tubeprovides physical isolation of the electrode and inner electrolyte fromthe cell electrolyte while serving as a membrane while the metal tubeprovides a shield and basket over the glass tube. Under the conditionsof high temperature, motion of the rotating stirrer in the cellelectrolyte and other operating conditions in the cell, the elongatedglass membrane may be subject to breakage. If glass from a brokenmembrane is allowed to come in contact with a uranium-containing anodepool, the amount of uranium available for transfer to the cathode willbecome depleted by the formation of uranium dioxide. Accordingly, themetal tube is designed to provide protection to the glass tube whilealso serving as a basket to collect separate glass sections of the tubebefore they would fall into the cell electrolyte and subsequently into alower anode pool. Perforations in the side wall of the metal tube permitaccess between the glass membrane and cell components.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a top view of an electrolytic cell with various cellcomponents including a reference electrode.

FIG. 2 is a side sectional view of the cell of FIG. 1 taken along line2--2' showing a reference electrode.

FIG. 3 is a sectional side view of a reference electrode as oneembodiment of the invention.

FIG. 4 is a side view of the protective metal tube showing perforationsin the side walls.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The reference electrode of the invention is particularly useful in hightemperature electrolytic cells where glass membrane sections may bebroken during removal or replacement operations in the top cover or lidof the cell housing or by thermal shock at the high temperatures. Thesecells usually include a cell housing of a rigid metal constructioncontaining a salt electrolyte and a pair of electrodes. In oneexperimental cell for the recovery of uranium from a mixture of metalswhere the reference electrode has been tested, the cell interiorincludes a mixed halide salt as the electrolyte over a cadmium pool asthe anode with the cathode being a solid metal rod suspended into theelectrolyte. The design further includes a motor for rotating thecathode and devices for separately stirring the electrolyte and anodepool. The operating temperature for the cell is usually in the order ofabout 500° C. The cell and reference electrode of the invention operatein an inert atmosphere of argon with less than 2 ppm of water vapor andless than 2 ppm of oxygen.

A top and sectional side view of a cell 8 of this type is illustrated inFIGS. 1 and 2. The cell housing 10 includes circular side wall 12,bottom 14 and upper cover 16 which serves to support drives for cathoderotation, electrolyte stirring and anode stirring. As illustrated, thecathode 18 of low carbon steel is rotated by drive 20 while theelectrolyte 22 is stirred by drive 24. Electrolyte 22 is composed of amixture of alkali and/or alkaline earth metal chlorides such as Li, K,Ba and the like and preferably is a mixture of LiCl and KCl in a moleratio of about 3:2. Stirring of the cadmium pool 26 is provided by motordrive 28. In general the drives are operated at speeds in the order of100-200 rpm. The construction of housing 10 is low carbon steel. An ironinsert 30 and plug inserts 32 and 34 are provided to adjust the cadmiumto the desired level in the housing while permitting some excesscapacity to the extent desired.

FIGS. 1 and 2 also include reference electrode 40 mounted in cover 16and extending down into electrolyte 22. Advantageously, referenceelectrode 40 is positioned near cathode 18 but separated a distance toavoid being struck by the rotating metal deposition 44 on the cathode.As illustrated, reference electrode 40 includes an upper cover 41 toprevent contaminants from falling into the electrode. The celltemperature is in the order of about 500° C. which is provided by anelectric furnace (not shown).

The reference electrode of the invention is useful for mounting in ahousing of an electrolytic cell and extending into an electrolyte layerin the interior of the cell, the device comprising an elongated glasstube of one-piece construction extending from the housing into theinterior of the cell and having a lower closed-ended section with ionicconductivity for exposure to the cell electrolyte, a metal electrodesupported in the glass tube and in separated juxtaposition with theclosed end, an electrolyte in the lower section in contact with theelectrode, and an electrical insulator separating the electrode andglass tube. The device further includes an elongated metal tubeextending from the housing over the glass tube as a protective cover andbasket with the metal tube having a perforated side wall in contact withthe cell electrolyte, and means are provided for supporting the glasstube within the metal tube and for supporting the metal tube in the cellhousing.

Advantageously, the metal tube is electrically isolated from the cellhousing to reduce the adverse effect of cell voltages and currents onthe operation of the reference electrode. It is also important thatmeans are provided for supporting and electrically insulating theelectrode from the glass tube and an alumina tube may be advantageouslyused for that purpose. A further feature of advantage is a removablecover over the upper open end of the glass tube to prevent contaminationof the contents of the tube.

FIG. 3 represents a sectional side view of a reference electrode 50enlarged to provide additional detail. As illustrated, referenceelectrode 50 includes an inner electrode of silver wire 52 supported inan alumina tube 54 which isolates electrode 50 from glass tube 56.Retention of electrode 52 in the alumina tube is provided by slot 60 inthe lower end 58 of tube 54 which serves to capture a looped bentportion 62 of the electrode. The upper end 64 of the electrode is alsobent to prevent the electrode from moving downward in the tube. Anelectrolyte 66 composed of AgCl plus a mixture advantageously of thecell electrolyte such as LiCl and KCl is also in the tube.

Advantageously, glass tube 56 is constructed of a high strength glasshaving a high silica content which will include a small alkali metalcontent for ionic conductivity or have the property of absorbingsufficient salt from the electrolyte for the desired conductivity.Suitably, the glass has a silica content above about 90 wt. % andpreferably a content in the order of 96 wt. %. Advantageously, the glassmay be a type identified by the trademark "Vycor" from Corning GlassWorks of Corning, N.Y. having a silica content of about 96 wt. % withthe remaining percentage consisting essentially of B₂ O₃, Al₂ O₃ andcompounds of Na and Fe. Quartz glass may also be used. Tube 68 isconstructed of low carbon steel and is designed to extend over glasstube 56 as a shield and basket. An insulating sleeve 70 of alumina ispositioned over a portion 71 of tube 68 to electrically isolate tube 68from the cell housing 10. Nut 72 is bonded to the alumina sleeve 70 andthreaded to mount the device in the cover 17 of a cell housing. A cover74 is also provided over the reference electrode 50 to preventcontamination of the electrode and electrolyte in the glass tube 56.

As illustrated in FIGS. 3 and 4, metal tube 68 includes a lower section76 with small perforations 78 in the order of about 1/8 inch in diameteralong lower wall 80. Slot 84 at curved end 82 is provided for ease offabrication from open-ended tube material, although the spring fingers85 also provide some resiliency and support glass tube 56. Slot 84 is alimited opening of about 1/16th inch and effectively prevents the lossof the usual large glass sections from a damaged glass tube. Asillustrated, glass tube 56 is of one-piece construction and extends intothe interior of the housing with a lower end section 59 being supportedby curved end 82 of metal tube 68 to provide a slight compression on theglass. The upper section 61 of glass tube 56 may rest against metal tube68 since clearance (i.e., about 1/16") is small or may be positionedupright in metal tube 68.

An additional detail of a metal tube 86 is provided in FIG. 4 whichincludes upper section 88 with nut 90 and upper and lower rings 91 and92 for support of the alumina tube. In addition, metal tube 86 includeslower section 94 with perforations 96 and lower slot 98.

The foregoing description of embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electrolytic cellcomprising a housing, an anode, cathode and reference electrode andadapted to contain a molten-salt electrolyte above said anode, whereinsaid reference electrode comprises an elongated glass tube of one-piececonstruction extending from said housing into the interior of said celland having a lower closed-ended section with ionic conductivity, a metalelectrode supported in the glass tube and in separated juxtapositionwith the closed end, an electrolyte in the lower section in contact withthe electrode and an electrical insulator physically separating theelectrode and glass tube; an elongated metal tube extending from thehousing over the glass tube as a protective cover and basket, the metaltube having a perforated side wall adapted to be in contact with thecell electrolyte and being closed-ended with respect to collecting anybroken glass from the glass tube so as to act as a basket to minimizethe amount of broken glass from otherwise falling through theelectrolyte into the anode; means for supporting the glass tube withinthe metal tube and means supporting the metal tube in the cell housing.2. The device of claim 1 including a removable cover positioned over theupper end of the glass tube above the housing.
 3. The device of claim 1wherein the supporting means for the metal tube includes means forelectrically insulating the metal tube from the housing.
 4. The deviceof claim 1 wherein the insulator for the electrode includes means forsupporting the electrode in the glass tube.
 5. The device of claim 4wherein the insulator includes a lower end section with at least one endslot in which the electrode extends to prevent removal of the electrodeapart from the insulator.
 6. The device of claim 5 wherein the electrodeis silver, the electrolyte within the glass tube is a mixed salt of AgCland alkali and/or alkaline earth metal chlorides, and the metal tube isof ferrous metal.